LEARNING STATION III: WHAT OSCILLATES WITH LIGHT? 27 1 Mechanical waves 27 1.a Source of mechanical waves 27 1.b Medium needed? 27 1.c Propagation and displacement in the same direction or in a different direction? 28 1.d Do the particles travel along with the wave? 29 1.e The source of light waves 30 2 Intermezzo Sound: Are there vibrations that don’t repeat? 30 3 Light: what’s shaking? 33 3.a Force(fields) that can travel through empty space 33 3.b Fields that change in time: waves of a field 35 3.c Electromagnetic waves 36 4 A sea of electromagnetic waves 38 Attribution-NonCommercial-ShareAlike 4.0 International (CC BY-NC-SA 4.0) Under the following terms: Attribution — You must give appropriate credit, provide a link to the license, and indicate if changes were made. You may do so in any reasonable manner, but not in any way that suggests the licensor endorses you or your use. NonCommercial — You may not use the material for commercial purposes. You can: Share — copy and redistribute the material in any medium or format Adapt — remix, transform, and build upon the material The licensor cannot revoke these freedoms as long as you follow the license terms. You have to refer to this work as follows: Frans R., Tamassia L. (2014) Quantum SpinOff Learning Stations. Centre for Subject Matter Teaching, KHLim Katholieke Hogeschool Limburg, Diepenbeek, Belgium
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LEARNING STATION III: WHAT OSCILLATES WITH LIGHT? 27
1 Mechanical waves 27 1.a Source of mechanical waves 27 1.b Medium needed? 27 1.c Propagation and displacement in the same direction or in a different direction? 28 1.d Do the particles travel along with the wave? 29 1.e The source of light waves 30
2 Intermezzo Sound: Are there vibrations that don’t repeat? 30
3 Light: what’s shaking? 33 3.a Force(fields) that can travel through empty space 33 3.b Fields that change in time: waves of a field 35 3.c Electromagnetic waves 36
4 A sea of electromagnetic waves 38
Attribution-NonCommercial-ShareAlike 4.0 International (CC BY-NC-SA 4.0) Under the following terms:
Attribution — You must give appropriate credit, provide a link to the license, and indicate if
changes were made. You may do so in any reasonable manner, but not in any way that
suggests the licensor endorses you or your use.
NonCommercial — You may not use the material for commercial purposes.
You can:
Share — copy and redistribute the material in any medium or format
Adapt — remix, transform, and build upon the material
The licensor cannot revoke these freedoms as long as you follow the license terms.
You have to refer to this work as follows:
Frans R., Tamassia L. (2014) Quantum SpinOff Learning Stations. Centre for Subject Matter Teaching,
Will magnets also exert a force on each other if they are in a
vacuum? Yes/No
The magnetic force doesn’t need to be transferred through a medium. The magnet induces
a magnetic field. The area around the magnet will get a new physical property, the
magnetic field.
If a nail or such is placed in spot where the field is present , a force will transpire. This
force doesn’t need a medium, it is induced by the field itself and the force can be exerted
from a distance without contact .
ii) The electric field
You’ve almost certainly seen how you can attract your
hair to a (nylon) comb without making contact. Here
we’re dealing with an electric force and it is . There is
also a force that works from a distance through a field.
Your hair doesn’t need to make contact with the comb.
Between the hairs, there is an electric repulsive force
that also works without making contact!
There is an electric field around the comb and
between the hairs. And wherever there is an electric field, an electric force can occur that
works from a distance without making contact.
Quantum Spin Off 34
Quantum physics: the physics of the very small with great applications
iii) Other fields: gravitation
Besides the electric and magnetic field, there is maybe the most
obvious field of all: the gravitational field.
To what force is it the cause?
………………………………………………………………
In 1687, Newton already determined there was a universal
gravitational force between masses like the sun and the Earth for
example.
Is this also an example of a force that is the result of a field?
Yes / No
Does this force work in a vacuum or does it need a medium?
…………………………………………………………………
Do objects still fall in a vacuum?
……………………………………………………………………………………………………
Name 3 forces that are the result of a field, and that also work in a vacuum:
1. ………………………………………
2. ………………………………………
3. ………………………………………
iv) Field a fundamental concept!
Initially physicists struggled a lot with the idea of “actio in distans”, a force working from a
distance through ‘nothing’. But they fundamentally embedded this idea in the concept of a
“field”. Since then the concept of field never left physics again and fields are ever since at
the center of every physical theory: gravitation fields, electric and magnetic fields and
even quantum fields!
Fields can have sources. For example masses are
sources of gravitational fields, charges are sources of
electric fields and magnets or electrical currents are
sources of magnetic fields.
Around the source you get an area where the field is
present.
The presence of a field can be tested by placing a
‘test’-object in the field: for instance a test charge will
experience an electric force in the electric field.
The test-object will experience a force from a distance
due to the field. The force may decrease with distance,
but basically you’ll find the force in all points in the
area around the source. This is exactly why physicists
see the area around the source as a field.
An electric field arises around a CD that is electrically charged by rubbing. The charges present in the atoms of the paper clippings are attracted by this field. (Source photo: Wikipedia)